1 Technical Field of the Invention
The present invention relates generally to an improved structure of a spark plug which may be used in internal combustion engines of automotive vehicles, cogeneration systems, or gas pumps, and more particularly to such a spark plug which achieves a higher degree of air-tightness.
2 Background Art
Spark plugs used in cogeneration engines are usually subjected to an increase in temperature of a seat of the spark plug (i.e., a gasket) due to the lack of cooling of the spark plug as compared with those used in automotive engines. The seat of the spark plugs is heated up to 200° C. to 300° C. Further, the cogeneration engines are usually higher in compressibility than the automotive engines, so that the spark plugs are exposed to compressed gas which will be higher in combustion pressure than that in the automotive engines.
Typical spark plugs have sealing powder filled in an annular chamber formed between a housing (i.e., a metal shell) and an insulation porcelain. The housing has an annular sleeve cold-crimped to compress the sealing powder to eliminate a clearance between the housing and the insulation porcelain in order to improve air-tightness therebetween. In some of the spark plugs, the annular sleeve is hot-crimped after cold-crimped in order to enhance the air-tightness further. For instance, Japanese Patent First Publication No. 11-242982 teaches such technique.
However, in recent years, the cogeneration engines have been required to improve the air-tightness of the spark plugs because of higher combustion pressure required for increasing the efficiency of burning of the engines.